ZnAl2O4-seeded CoAl2O4, with a core-shell structure, has been prepared under hydrothermal conditions when the Co2+ salt solution is substituted by 10% Zn2+ as a precursor. The ZnAl2O4 seed is generated during the synthesis process. The seeding process can decrease the synthesis temperature from 245degrees to 230degreesC and the particle size from 67 to 20 nm. The process can economize the consumption of Co2+ and control the particle size effectively.